The following discussion is not an admission that anything discussed below is common knowledge or citable as prior art.
Immersed membranes are used, for example, for filtering water in municipal water supply treatment plants and in wastewater treatment plants. The membranes may be microfiltration or ultrafiltration membranes made in the form of modules of hollow fiber membranes. In large plants, many modules are immersed in the water to be filtered in an open tank and filtered water, also called permeate, is withdrawn through the membranes by suction. In a typical filtration cycle, a period of permeation, for example for 15 minutes to an hour, is followed by a relatively short period of backwashing or relaxation, and then the cycle repeats. In wastewater plants, and also in some water supply plants, the membranes are scoured with bubbles during permeation to clean or inhibit fouling in the membranes. The bubbles are provided according to an aeration regime that is applied throughout most, and possibly all, of the permeation part of the filtration cycle, and possibly during the backwash or relaxation part of the cycle as well. The aeration regime uses a significant amount of energy, adding to the cost and environmental impact of the plant. Accordingly, there have been various attempts to reduce the energy required by the aeration regime while still maintaining an acceptable fouling rate in the membranes.
A cyclic aeration system is described in International Publication Number WO 2000/021890, published Apr. 20, 2000. In a version of this system used by GE Water and Process Technologies with its ZEEWEED™ hollow fiber membrane modules, air from one or more blowers is split through a set of controlled valves into two or four sets of aerators. The valves distribute the air to the sets of aerators in turn. For example, with two sets of aerators the air may be sent first to one set of aerators and then to the other set of aerators. An assembly of filtering membrane modules are connected together in a frame to provide a larger unit called a rack or cassette. There are typically several aerators below a cassette, but all of these aerators are part of the same assembly of aerators. In this case, the cassette experiences an aeration regime wherein bubbles are provided for a period of time, and then not generally provided for a period of time, in repeated cycles. For example, with two sets of aerators, bubbles may be provided to a cassette for 10 seconds out of every 20 seconds. This is also called a 10 seconds on, 10 seconds off cycle, or simply a 10, 10 cycle. With four sets of aerators, bubbles may be provided to a cassette for 10 seconds out of every 40 seconds, or in a 10, 30 cycle. In some plants, the valve set allows the system to switch between these two regimes depending, for example, on variations in the fouling qualities of the water or the feed flow rate.
In the MEMPULSE™ system by Siemens Water Technologies Corp., each module is provided with an integrated pulsing air lift pump. Each module in a rack or cassette has an air tube connecting its air lift pump to a common air supply pipe. The air tube discharges air into an inverted cup in the pulsing air lift pump. A pocket of air grows inside the cup until it extends downwards to a critical elevation, at which point most of the air in the pocket discharges into the adjacent vertical passageway. The passageway is open at its bottom end to draw in water in the tank, and its top is open to gaps in a lower potting head of the module. An air lift is formed in the passageway as the air rushes through, and bubbles and water are discharged through the gaps in the lower potting head into the module. The module experiences an aeration regime that, as in the cyclic aeration system, consists essentially of an air on—air off cycle. The precise timing of the cycle in commercial systems is not known to the inventor, but the related International Publication Number WO 2008/153818, published Dec. 18, 2008 suggests a cycle of roughly 2 seconds of bubbles, with entrained water, followed by 8 seconds without bubbles.